A collection of minor improvements

This slightly unwieldy PR collects multiple improvements mostly dealing with renaming of various classes and improved documentation. The following lists the changes:
- Avoid segfault reported as issue #8 by making sim_2p_comp_ad refuse to run without a deck.
- Add more helper matrices to the HelperOps class: grad, fullngrad and fulldiv.
- Rename AutoDiff::Forward -> Opm::AutoDiff.
- Rename AutoDiff::ForwardBlock -> Opm::AutoDiffBlock.
- Move contents of AutoDiffHelpers.hpp to Opm namespace.
- Documented Opm::AutoDiff.
- Documented Opm::AutoDiffBlock.
- Created main doxygen documentation file and added Doxylocal.